1. Field of the Invention
This invention relates to a membrane switch unit of click action type.
2. Description of the Related Art
A click-action type membrane switch unit used on a solid printed circuit board has a number of advantages: it can be operated with high reliability, it can be formed thin, it occupies only a small amount of space and it can be manufactured at low cost. Because of these advantages, this type of switch unit is widely used in the operating key portions of electrical equipment and the like. In particular, the click-action type membrane switch unit is frequently used in printed circuit boards as a switch unit which has a large number of key input portions and can endure high temperatures.
A click-action type membrane switch can also be used in a flexible circuit board.
FIGS. 1 and 2 show a conventional click-action type membrane switch unit which is used in a flexible printed circuit board and comprises a base film 1, upper circuits 2a and 2b and a lower circuit 3 formed on the upper and lower surfaces of the base film 1, resist films 4 and 5 formed on the upper and lower circuits 2a, 2b, and 3, electrode contacts 6 extending from each upper circuit and arranged in an interdigitating manner in a circular window 7 formed in the intermediate portion of the resist film 4 and a dome-shaped click spring 8 made of metal having a strong spring force or of any other material having electrical conductivity and covering the electrode contacts 6 and the window 7.
Referring to FIG. 4, when the click spring 8 is depressed from its stationary state, as indicated by a broken line 9, its central portion is deformed by an amount indicated by A and contacts the electrode contacts 6 of the upper circuits 2, as indicated by a broken line 10, whereby the upper circuits 2a and 2b are electrically connected. When the click spring 8 cannot be depressed more than A, a load required for this displacement A is represented by SA in FIG. 5, which does not provide sufficient feeling to the operator, as indicated by the broken line 14 in FIG. 5.
In order to produce sufficient clicking sensation, however, it is required that the click spring 8 be deformed further by an amount B (i.e., the total displacement indicated by C), as shown by a solid line 12 in FIG. 4. In this case, a load SB is required for the displacement B, as shown in FIG. 5. In an ideal case, the load-displacement characteristic as indicated by a solid line 13 in FIG. 5 is required of a normal click spring, wherein an ample displacement C occurs under a load Sc, which is the sum of the loads SA and SB.
When the click spring 8 of the conventional click-action membrane switch unit is depressed, it is deformed and its central portion contacts the upper surfaces of the electrode contacts 6. Because the switch unit is usually placed on a rigid base, however, the central portion of the click spring 8 cannot be depressed below the level 11 shown in FIG. 4, as a result of which the click spring 8 assumes the form shown by the broken line 10, wherein the degree of depression or the sinking of the click spring 8 is less by the amount B than in the ideal case, resulting in the very poor load-displacement characteristic 14 shown in FIG. 5, as compared with the ideal load-displacement characteristic 13 shown in this figure.
In view of this, the conventional click-action type switch unit has the drawback that it cannot provide the operator with sufficient clicking sensation.
In order to increase the clicking sensation, a click-action type membrane switch unit having a click spring provided with an increased squeezing force for enlarging the displacement thereof was thought of. However, this type of switch unit requires a thicker click spring, which entails drawbacks such that it is no longer possible to form the switch unit thin and the substantial increase in the maximum load applied to the click spring degrades the durability of the switch unit. These are fatal drawbacks when the switch unit is to be miniaturized.
FIG. 3 shows a conventional click-action membrane switch with lighting, which is an application of the click-action membrane switch unit of the conventional type as shown in FIGS. 1 and 2. This switch comprises a base film 1, upper and lower circuits 2 and 3, resist films 4 and 5, electric contacts 6, a circular window 7, and a click spring 8, all arranged similarly to the conventional switch unit shown in FIGS. 1 and 2. The upper resist film 4, a light-loading and diffusing sheet 15, a formed rubber sheet 16 and an outer sheet 17 are laminated one on another. Formed in the light-loading and diffusing sheet 15 and the formed rubber sheet 16 are holes 18 and 19 situated adjacent to each other. The hole 18 houses the click spring 8, and the hole 19 houses a lighting unit 20 comprising an LED bare-chip element 21 mounted on one of the upper circuits 2 and electrically connected to the other upper circuit 2 by means of a bonding wire 22.
The above click-action type membrane switch with lighting also encounters the same problems as the switch unit shown in FIGS. 1 and 2, and described above.